Typically known continuously variable transmissions (CVT) include a variety of types such as a metallic belt type, a chain type and a traction drive type. Any type of CVT is required to provide a high power transmission capacity. Performance of the CVT depends on features of a lubricating oil to be used, i.e., a friction coefficient between metals or a traction coefficient. The larger both coefficients become, the higher the power transmission capacity becomes.
Accordingly, various lubricating oil compositions for the continuously variable transmissions that can favorably transmit power with a large capacity are known (see, for example, Patent Documents 1 to 5).
Patent Document 1 discloses that a sulfuric extreme pressure agent, a phosphorus extreme pressure agent and an alkaline earth metal detergent are added to a lubricating base oil so as to provide excellent wear resistance and extreme-pressure property, retain a high friction coefficient for a long period of time and transmit a large amount of torque.
Patent Document 2 discloses that a calcium sulfonate having a total base number of 50 to 100 mg KOH/g and a phosphite ester are added to a base oil so as to prolong a life of an anti-shudder property (shudder is vibration occurring at a time of lockup) in an automatic transmission equipped with a slip-lockup mechanism and enhance a long-lasting life of a scratch-noise prevention property in a belt-type continuously variable transmission.
Patent Document 3 discloses that a metal detergent such as calcium and a phosphorus anti-wear agent are added to a lubricating base oil so as to provide a high friction coefficient between metals and an excellent anti-friction property to a belt and a pulley in a push-belt-type continuously variable transmission. The metal detergent is added so that a ratio between a metal content in a total weight of a lubricating oil composition and a total base number is in a range of 0.75 to 4.5. The phosphorus anti-wear agent is added so that a ratio between a content of metal such as calcium derived from the metal detergent and a phosphorus content is in a range of 0.5 to 2.0.
Patent Document 4 discloses that a phosphate ester having an alkyl group containing a thioether bond and one or more selected from the group of a phosphate ester, an amine salt thereof and an overbased calcium sulfonate are added to a base oil so as to improve and stabilize a power transmission capacity, the base oil including a hydrocarbon compound having a cohesive energy density of 0.180 GPa or more at 40 degrees C. to have a kinematic viscosity of 5 to 150 mm2/s at 40 degrees C.
Patent Document 5 discloses that a lubricating base oil, a phosphorus compound, and at least one organic acid salt selected from the group of an alkaline earth metal sulfonate, an alkaline earth metal phenate and an alkaline earth metal salicylate are mixed so as to retain a sufficient wear resistance and a high anti stick-slip property for a long period of time. A content of the phosphorus compound in a total amount of a composition is in a range of 0.01 to 0.2 (in terms of phosphorus), a content of the organic acid salt in the total amount of the composition is in a range of 0.01 to 0.2 (in terms of alkaline earth metal) and a ratio of phosphorus to alkaline earth metal is in a range of 0.1 to 10.    Patent Document 1: JP-A-9-100487    Patent Document 2: JP-A-10-306292    Patent Document 3: JP-A-2001-342485    Patent Document 4: JP-A-2005-281474    Patent Document 5: JP-A-2006-152092